Hybrid vocoder spectrum expander



March 30, 1965 T. M. STUMP 3, 7 5

HYBRID VOGODER SPECTRUM EXPANDER Filed Sept. 25, 1961 EOUT 2 3 E i W OUTPUT I l IN V EN TOR. THEODORE M. S 7' UMP AQA JJ Wm ATTORNEY United States Patent M 3,176,155 HYBRID VOCODER SPECTRUM EXPANDER Theodore M. Stump, Webster, N.Y., assignor to General Dynamics Corporation, Rochester, N .Y., a corporation of Delaware Filed Sept. 25, 1961, Ser. No. 140,490 6 Claims. (Cl. 30788.5)

The present invention relates to spectrum expanders utilized in hybrid vocoder receiving circuitry.

In hybrid vocoder transmission systems, information contained within the lower portion or base-band of the frequency spectrum is directly transmitted to the receiver whereas information within the intermediate and upper portions of the spectrum is coded at the transmitter and sent to the receiver by a multiplex process as a plurality of discrete signals, each signal containing information as to the energy content of an associated sub-band within the intermediate and upper frequency spectrum. Each of these signals controls the degree of modulation of a corresponding frequency generated at the receiver which may be produced by a spectrum expander. The input circuit of the spectrum expander is conventionally coupled to the transmission link through a filter which passes frequencies withinthe base-band. The output circuit of the spectrum expander is coupled to a plurality of modulators, each one corresponding to an associated sub-band, through band-pass filters so that the frequency components in the intermediate and upper frequency spectrum are generated by the. spectrum expander andapplied to these modulators. If the energy within the base-band is produced from sounds using vocal cords the frequencies generated by the spectrum expander will be harmonics of the fundamental vocal cord frequency within the intermediate and upper frequency spectrum. On the other hand, if the energy within the base-band is produced by fricative sounds, the spectrum expander will generate numerous frequencies having energies more or less evenly distributed within theintermediate and upper frequency spectrum. It is known that unvoiced sounds e.g., hissing sounds, produce signals having the latter characteristic, while other voiced sounds, e.g., vowel sounds, produce signals having the former characteristic. In earlier prior art vocoders, a buzz-hiss decision circuit was required to control circuitry at the receiver for generating the frequencies in the intermediate and upper frequency bands. In addition, prior art vocoders utilized a pitch-tracking filter to latch onto the pitch frequency to thereby control the buzz circuit at the receiver which generated the requisite frequencies in the intermediate and upper frequency bands. These systems were complicated and frequently latched onto the incorrect pitch frequency, such as the ubiquitous sixty-cycle signal.

Recently, a so-called zigzag spectrum expander was developed which did away with the aforementioned disadvantages. This circuit produced the appropriate frequencies within the intermediate and upper portions of the spectrum regardless of whether or not the base-band signals were buzz-type signals or hiss-type signals. No tracking filter is required With this circuitry.

However, it has been found that variations in the amplitudes of the frequency components applied to the input circuit cause undesirable variations in the various frequencies produced in the output circuit. These variations in amplitude of the input frequencies occur due to the fact that certain sounds normally produce frequency components having relatively small amplitudes when compared with the amplitudes of frequency components produced by other sounds. These variations are not compensated for by the relatively slow-acting gain control circuit normally inserted in the transmission path 3,176,155 Patented Mar. 30, 1965 to compensate for changes in the strength of the over-all voice level. Such variations introduce distortion in the final complex wave reproduced by the adder circuit, normally coupled to the output circuits of the modulators at the receiver.

In addition, the zigzag spectrum expander operates in a hybrid vocoder having a broad base-band, such as 250- 2280 cycles per second (with the spectrum extending to 10,000 cycles/second) in contrast to the spectrum expander of the present invention which operates in a hybrid vocoder utilizing a base-band of 250-750 or 60-300 .cycles per second (with the spectrum extending to 3250 cycles/second). As a result, a greater amount of noise will be present in the vocoder system utilizing the zigzag circuit in contrast with a vocoder system utilizing the spectrum expander of the present invention.

Accordingly, it is the principal object of the present invention to produce a new and improved spectrum expander for vocoder operation which eliminates numerous disadvantages present in prior art expanders.

It is a further object of the present invention to provide a new and improved spectrum expander which produces the requisite frequency components for vocoder operation in the intermediate and upper portions of the frequency spectrum regardless of variations in the amplitudes of the frequency components transmitted in the base-band.

Further objects and advantages of the invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawing in which:

FIGURE 1 discloses a representation of the output characteristics of the aforementioned zigzag spectrum expander;

FIGURE 2 discloses a preferred embodiment of the present invention; and

produce a voltage wave shape 3, which wave shape is rich in higher order harmonics which will produce various frequency components in the intermediate and upper portions of the frequency spectrum mentioned hereinbefore. In the event that signal 1 is not sinusoidal but is a complex wave shape, the output wave shape 3 will, of course, differ in form from the wave shape as shown. However, it has been found that a spectrum expander having the zigzag characteristic curve of FIGURE 1 will generate the necessary higher order frequency components for proper hybrid vocoder operation. However, should the input signal have a reduced amplitude, such as that of input signal 4, it will be apparent that the requisite wave shape, represented by 3, will not be produced by the zigzag spectrum expander of the prior art. However, the spectrum expander of the present invention will produce the requisite wave shape 3 where the input signal is sinusoidal regardless of the amplitude of the input signal.

In the preferred embodiment of the present invention disclosed in FIGURE 2, an input signal, as represented by wave shape A of FIGURE 3, is applied to transformer 6 and is rectified by full wave rectifier 7 to produce wave shape B of FIGURE 3. The DC. component of wave shape B is removed by high-pass filter 8 to produce wave shape C of FIGURE 3, which, in turn, is reapplied to the input circuit of full Wave rectifier 9. As a result of further full wave rectification by rectifier 9, wave shape D. is produced. It should be noted that portions 11 of wave shape D will have peak amplitudes in excess of the peak amplitudes of portions 12 so that the desired waveform'3 of FIGUREl is not attained when rectifier is balanced. The reason for this is that the volt-second integral of a full cycle of waveform C of FIGURE 3 must be equal to zero since the DC. component is removed by high-pass filter 8. It is, therefore, evident upon inspection that the absolute value of the peak voltage represented by 13 of FIGURE 30 will be greater than the absolute value of the peak voltage represented by 14' of FIGURE 3C. However, if waveform 3C is shifted in an upward direction relative to the zero access to the point where the absolute value of voltage 13 is equal to the absolute value of voltage 14, the full wave rectification of this Wave shape will produce wave shape D of g applying substantially all of the frequencies produced by said second full-wave rectifier to said utilization device, and means for unbalancing said first full-Wave rectifier.

2; In a spectrum expander for applying a plurality of frequencies to a utilization device, a signal source, a first and second full-wave rectifier, means for coupling said signal source to the input circuit of said first full-Wave rectifier, means for applying only the AC. component of the unfiltered output of said first full-wave rectifier to the input circuit of said second full-wave rectifier, means for applying substantiallyall of the frequencies produced by said second full-Wave rectifier to 'said utilization device, and means for unbalancing said second full-wave rectifier.

3. In a spectrum expander for applying a plurality of frequencies to a utilization device, a signal source, a

, first and second full-wave rectifier, means for coupling FIGURE 3, which is the required wave shape. This base-band on the order of 60-300 cycles per second, for 7 example, couldbe expanded properly.

It should be evident from the foregoing explanation that regardless of the amplitude of a sinusoidal input signal applied to transformer .6, therequisite wave shape D of FIGURE 3 will be produced. a 7

While there. has been disclosed what is at present considered to be the preferred embodiment of the invention, other modifications will readily occur to those skilled in the art. It is not, therefore, desired that the invention means for applyingonly'the A.C. component of the unsaid signal source tothe input circuit of said first full- Wave rectifier, means for applying only the A.C. component of the unfiltered output of said first fullewave rectifier to the input circuit of said second full-Wave rectifier, means for applying substantially all of the frequencies produced by said second full-wave rectifier to said utilization device, and means for unbalancing said first and second full-wave rectifier.

4. The combination as set forth in claim 1 wherein said means for applying only the. AC. component of the unfiltered output ofs aid first full-wave rectifier to the input circuit of said second full-wave rectifier comprises a high-pass filter.

5. The combination as set forth in claim 2 wherein said filtered output of said first full-wave rectifier to the input be limited to the specific arrangement shown and described, and it is intended in the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What is claimed isi V 1. In aspectrum expander for applying a plurality of frequencies to a utilization device, a signal source, a first and second full-wave rectifier, meansfor coupling said pass filter. p r 6. The combination as set forth in claim 3 wherein said means for applying only the A.C. component of the unfiltered output of said first full-wave rectifier to the input circuit of said second fullswave rectifier comprises a high-pass filter. a

. References Cited bythe Examiner UNITED STATES PATENTS 10/43 Schlegel r 32ll0 X 2,418,114 1/47 Frankel -4 32110 2,978,642 4/61 Papineau "328-20 3,044,004 7/62 Sic'ard' 321 -69 X ARTHUR GAUSS, Primary Examiner. JOHN W. HUCKERT, Examiner. 

1. IN A SPECTRUM EXPANDER FOR APPLYING A PLURALITY OF FRQUENCIES TO A UTILIZATION DEVICE, A SIGNAL SOURCE, A FIRST AND SECOND FULL-WAVE RECTIFIER, MEANS FOR COUPLING SAID SIGNAL SOURCE TO THE INPUT CIRCUIT OF SAID FIRST FULL-WAVE RECTIFIER, MEANS FOR APPLYING ONLY THE A.C. COMPONENT OF THE UNFILTERED OUTPUT OF SAID FIRST FULL-WAVE RECTIFIER TO THE INPUT CIRCUIT OF SAID SECOND FULL-WAVE RECTIFIER, MEANS FOR APPLYING SUBSTANTIALLY ALL OF THE FREQUENCIES PRODUCED BY SAID SECOND FULL-WAVE RECTIFIER TO SAID UTILIZATION DEVICE, AND MEANS FOR UNBALANCING SAID FIRST FULL-WAVE RECTIFIER. 